Connecting the same device via both WiFi and ethernet is not advisable. You can create a lot of interesting, not mention difficult to troubleshoot, network conflicts that way. But there is no conflict if some devices are WiFi only and others ethernet only. I have a variety of both on my LAN.
Wifi is convenient because the devices are not "tethered" to an expensive ethernet cable but ethernet is easily faster, more reliable, and because of its greater bandwidth less affected by traffic load. If it weren't for the effort and cost of running ethernet cable all of my desktop computers and devices would be connected via ethernet. Just remember, your internet speed is limited to what your ISP provides which is almost certainly slower than either WiFi or Ethernet.
